1. Field of the Invention
This invention relates to a method of manufacturing a tubular vibration-insulating bushing, and more particularly to such a method which obviates the need for a drawing step which requires large-scale facilities, thereby greatly reducing the manufacturing cost.
2. Description of the Related Art
A conventional vibration-insulating bushing of the type under consideration comprises an inner tube, an outer tube mounted around the inner tube and coaxially therewith, and a vibration-insulating rubber member formed between and integrally joined to the inner and outer tubes. Such a bushing has been extensively used as an engine-mounting member and a suspension bushing.
In accordance with the general method of manufacturing such a vibration-insulating bushing, the inner and outer tubes are placed in a mold, and a rubber material is poured into the space between the inner and outer tubes so as to mold a vibration-insulating rubber member which is integrally joined to the inner and outer tubes through vulcanization. When the vibration-insulating rubber member is molded, a tensile stress develops in the rubber member because of thermal shrinkage. This decreases the durability of the rubber member. For that reason, conventionally, the outer tube has been drawn into a smaller diameter after the molding of the vibration-insulating rubber member, so as to eliminate the above-mentioned tensile stress and also to apply a slight compression to the bushing.
The term "pouring" as used above means to supply the molding material into a closed space within the mold from outside the mold, and should be broadly construed and includes "injection".
The vibration-insulating bushing thus manufactured is press-fitted into and fixed to the cross-sectionally circular bore of a holder bracket.
In one example of such vibration-insulating bushings, the vibration-insulating rubber member is disposed within the outer tube to extend widthwise transversely of the outer tube so as to reduce the spring constant in one direction perpendicular to the axis of the bushing, and the inner tube is embedded in and extends through the central portion of the rubber member.
Usually, the outer tube is made of steel, and the drawing rate needs be about 5 to 10% (About 2% is the shrinkage strain of the vibration-insulating rubber member). Therefore, drawing equipment is relatively large, and considerable cost and time are required. Further, with respect to the press-fitting of the outer tube, the amount or rate of pressing must be accurately determined. Otherwise, the outer tube would be subjected to strain, or the fixing force would be inadequate. To avoid these problems, it has been necessary to carefully determine and carry out press-fitting with a great deal of labor and time.